The present invention generally relates to improvements in transformers and, more particularly, to an improved low-voltage transformer for use in landscape lighting applications.
Transformers are well known electrical devices that utilize a primary and secondary winding to provide an output potential from the secondary winding that is higher or lower than the potential of a power source coupled to the primary winding. One particular application for such transformers is in landscape lighting systems, which typically employ low-voltage landscape lighting fixtures. Low voltage landscape lighting systems are safe, economical, energy efficient and provide numerous benefits for modem homeowners. Lighting can be used to provide safe access near paths, drives and entry areas. Outdoor lighting increases security by discouraging potential intruders. And the beauty of garden and home can be dramatically enhanced by showcasing architectural and plant features with dramatic lighting techniques. Low voltage systems have become more and more popular, and offer the advantages of easier cable installation without burdensome conduits, reduced risk of electrical shock and lower power consumption when compared with typical high-wattage 120 volt (V) lamps.
In landscape lighting applications, the primary winding of a landscape lighting transformer is coupled to a 120V power source and the secondary winding is connected to one or more runs of 12V landscape lighting fixtures. Some landscape lighting transformers incorporate a plurality of taps to enable a range of output voltages for different wire runs that require higher voltages to compensate for power losses through the wire runs as a result of excessive wire lengths or to control lamp brightness. In such cases, the transformer may have terminals corresponding to a range of voltages such as, for example, from 12V to 18V. Halogen lamps that are conventionally utilized in such fixtures have an acceptable operating range of between 10.8V to 12V, with an optimal operating range of between 11.0V to 11.5V. As a general industry practice, low-voltage landscape lighting transformers are manufactured with power ratings in multiples of 300 watts (W). To protect the transformer from overload or damage, the secondary winding typically includes one or more circuit-breakers or fuses. In conventional transformers, the primary winding is not protected by a circuit-breaker or fuse, since the circuit-breaker(s) on the secondary winding are usually sufficient to protect the core from overloading.
Prior art transformers suffer a drawback in that the full power capacity of the transformer is typically not accessible. Notably, one major design flaw in a conventional landscape lighting transformer is that, on average, 10–20 percent of the wattage capacity of the transformer is often left unutilized. This is because an installer simply cannot use 100% of the current capacity of the secondary winding. For example, the secondary winding of a 300 W transformer may have a single common tap protected with a 25 amp (A) circuit breaker. In an exemplary application, assume that a given wire run of lamps rated at 90 W each are operating at 12V. Accordingly, each lamp/wire run draws a current of 7.5A in accordance with the well known principle P=iV (e.g., 7.5 A=90 W/12V). Thus, only three lamp/wire runs (which draw a total of 7.5 A×3=22.5 A), can be coupled to the secondary winding because any additional lamp/wire run would draw current in excess of the 25 A breaker limit. Accordingly, using this example, 2.5 A (e.g., 25 A–22.5 A) of unused capacity remains on the common tap, which equates to 30 W of unused power at 12V (e.g., 2.5 A×12V). This amounts to 10% of the transformer's total power capacity. Although 10% may not seem like a significant amount, the greater the power capacity of the transformer, the larger the power capacity that may be left unutilized. Thus, although a customer may have purchased a transformer having a specified power output, such as 300 W, under the scenario discussed above, 30 W of unused power is not accessible and therefore wasted.
Conventional landscape lighting transformers are typically manufactured with power ratings in multiples of 300 W, with associated common taps in multiples of 300 W. In a prior art 1,200 W transformer, therefore, four common taps are provided, one for each 300 W. In a 900 W transformer, three common taps are provided. In a 600 W transformer, 2 common taps are provided. These transformers suffer from drawbacks in that the full power capacity of the transformer can never be realized as explained above. For example, to fully utilize the full 1,200 W of a 1,200 W rated transformer, wire runs that add up to exactly 300 W for each of the four taps would have to be employed. In reality, this never happens as a typical installation will leave unused capacity on each tap. Thus, even though the load of the initial wire runs may only be 900 W, if it is desired to add another run of 200 W, which is still within the overall capacity of the transformer, it may be impossible to do so because of insufficient capacity remaining on any common tap.
In view of the above, there exists a need for an improved transformer that enables an installer of landscape lighting fixtures to take advantage of the full rated capacity of the transformer.